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EPA/540/S-97/505

January 1998

United States

Environmental Protection

Agency

Office of Solid Waste

and Emergency

Response

Office of

Research and

Development

Ground Water Issue

Steam Injection for Soil and Aquifer Remediation

Robert S. Kerr Environmental

Research Laboratory

Ada, Oklahoma

Superfund Technology Support Center for

Ground Water

* U.S. Environmental Protection Agency

National Risk Management Research Laboratory

Subsurface Protection and Remediation Division

P. O. Box 1198, Ada, OK 74820

Eva L. Davis

Technology Innovation Office

Office of Solid Waste and Emergency

Response, US EPA, Washington, DC

Walter W. Kovalick, Jr., Ph.D.

Director

Background

Innovative technologies for subsurface remediation, including

in situ techniques based on heating the subsurface to enhance

the recovery of organic contaminants, are increasingly being

evaluated for use at specific sites as the limitations to the

conventionally-used techniques are recognized. The purpose

of this Issue Paper is to provide to those involved in assessing

remediation technologies for specific sites basic technical

information on the use of steam injection for the remediation of

soils and aquifers that are contaminated by volatile or semivolatile

organic compounds. A related Issue Paper, entitled “How Heat

Can Enhance In-Situ Soil and Aquifer Remediation: Important

Chemical Properties and Guidance on Choosing the Appropriate

Technique” (Davis, 1997), discusses the properties of some

organic chemicals commonly found at contaminated sites, how

these properties are affected by the presence of the chemical in

a porous media, and how heat can enhance the recovery of these

chemicals from the subsurface. The Issue Paper also provides

information on three general types of heat-based remediation

systems, and some guidance on which technique is most

appropriate for different soil and aquifer conditions and for

different organic chemicals.

This document contains more detailed information on how

steam injection can be used to recover organic contaminants

from the subsurface, the contaminant and subsurface conditions

for which the process may be appropriate, and general design

and equipment considerations. In addition, laboratory and fieldscale experiments are described, and available treatment cost

information is provided. This document is not meant to provide

detailed information that would allow the design of a steam

injection remediation project, but rather design considerations

are provided to familiarize remediation workers with what is

involved in the process.

Introduction

Steam injection was first developed by the petroleum industry

for the enhanced recovery of oils from reservoirs. In petroleum

industry applications, steam is injected to lower the viscosity of

heavy oils and to increase the volatility of light oils. As much as

50 percent of the original oil in place may remain in the reservoir

when the process becomes uneconomical and is discontinued.

In the past several years, steam injection has been adapted for

the recovery of organic contaminants from the subsurface, and

extensive laboratory and field research has been done. When

steam injection is used for subsurface remediation, the objective

is to remove as much of the contamination as possible, thus

reducing the residual to very low levels. The subsurface conditions

dealt with by the petroleum industry versus remediation purposes

are generally very different - the petroleum industry dealing with

deep, confined reservoirs and the remediation industry with the

shallow, generally unconfined subsurface. Thus, the petroleum

industry technique and the technique for remediation purposes

differ in significant ways.

Basic Principles

Consider the situation shown in Figure 1 where steam is

injected surrounding a pool of a volatile contaminant in the

subsurface. The figure illustrates steam injection above the

water table, but steam injection for remediation purposes has

also been successfully carried out below the water table (Newmark

and Aines, 1995), and the basic principles described here are

the same for either situation. Initially, the steam that is injected

will heat the well bore, and the formation around the injection
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zone of the well. The steam condenses as the latent heat of

vaporization of water is transferred from the steam to the well

bore and the porous media where it enters the formation. As

more steam is injected, the hot water moves into the formation,

pushing the water initially in the formation (which is at ambient

temperature) further into the porous media. When the porous

media at the point of steam injection has absorbed enough heat

to reach the temperature of the injected steam, steam itself

actually enters the media, pushing the cold water and the bank

of condensed steam (hot water) in front of it.

As these flowing fluids approach a region that contains the

volatile contaminant at saturations greater than its residual

saturation, the contaminant is displaced. First to come into

contact with the contaminant is cold water, then the hot water

bank, and finally the steam front. The cold water will flush the

mobile contaminant (i.e., the contaminant saturation that is in

excess of its residual saturation) from the pores. The hot water

will reduce the viscosity of the contaminant, making it easier to

be displaced by viscous forces, and may reduce the residual

saturation of the contaminant. When the steam front reaches the

contaminated area, no additional contaminant can be recovered

by viscous forces. Additional recovery is achieved by

volatilization, evaporation, and/or steam distillation of the volatile

and semivolatile contaminants (Stewart and Udell, 1988).

Thus, when steam is injected into a porous media, three

distinct zones can be considered to develop: the steam zone,

the variable temperature zone, and the ambient temperature

zone. These three temperature zones are shown in Figure 2.

This figure also shows the relative concentrations of the

contaminants in each of the zones. Closest to the injection point

is the steam zone which, in the area close around the injection

point, is at approximately the temperature of the steam. Further

downstream within the steam zone, the temperature may

decrease somewhat due to heat losses to the overburden and

underburden. If the rate of steam injection is sufficiently high, the

percentage of heat lost will not be great, and this zone can be

considered essentially isothermal. In this zone, steam distillation

and steam stripping are the main recovery mechanisms and

these processes cause the formation of a contaminant bank just

downstream of the steam front (Wu, 1977). Downstream of this

is a variable temperature zone where condensation of the

injected steam and evaporation of the contaminants takes place.

The interface between the steam zone and the variable

temperature zone is essentially the location of steam

condensation, if the temperature gradient in the steam zone is

not significant. At the front of this zone is the bank of contaminants

that have been displaced by viscous forces, followed by the hot

water bank. The zone furthest downstream is the ambient

temperature zone which is saturated with water and the mobilized

contaminant.

The amount of residual saturation remaining after the cold and

hot water flush is dependent on the capillary properties of the

porous media, the interfacial properties of the contaminant, and

the pressure gradient in the water causing the displacement.

Residual organic liquid saturations in unconsolidated sands are

typically in the range of 14 to 30 percent (Wilson et al., 1990).

The texture of the soil has been found to be the most important

factor in determining the amount of residual organic contaminant

left in a soil. Several researchers have found that increasing the

temperature reduces the residual saturation of oils (Edmondson,

1965; Poston et al., 1970; Davis and Lien, 1993).

The residual contaminants are volatilized by the steam, and

the volatilized contaminants are transported to the steam front,

increasing the saturation of the contaminant in this zone and

adding to the contaminant bank. The rate at which the contaminant

bank formed by the evaporation and condensation processes

moves downstream is inversely proportional to the saturation of

the compound downstream, and directly proportional to the

volatility of the contaminant (Yuan, 1990). Experimental work

has shown that pure, separate, liquid-phase contaminants with

boiling points less than that of water will be completely removed

from the steam zone except for the small amount which is

adsorbed to solid surfaces or dissolved in liquid water which may

be present in the steam zone (Hunt et al., 1988). Theoretical

studies have predicted that under certain conditions, liquid

hydrocarbons having boiling points up to 175°C may also be

completely removed directly behind the steam condensation

front (Yuan, 1990; Falta et al., 1992b). This conclusion is

supported by one-dimensional column experiments which

Figure 1. Diagram of a possible steam injection remediation system.

Figure 2. The three temperature zones that form during a steam flood

(after Wu,1977).
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showed essentially complete removal of toluene and gasoline

(Hunt et al., 1988) and 96.8 to 99.8 percent recovery of No. 2 fuel

oil and jet fuel (Hadim et al., 1993) by steam injection.

Recovery Mechanisms

The first recovery mechanism acting on contaminants during

a steam injection is a physical displacement, first by the water

originally in the subsurface, then by the hot water formed by the

condensation of steam and finally by the steam itself. Physical

displacement of the contaminant occurs when there is an

immiscible organic liquid present at saturations greater than its

residual saturation, and when there is a dissolved phase in the

water that is displaced with the water. A physical displacement

of soil air occurs when steam is injected into the unsaturated

zone. When the initial contaminant concentration is significantly

greater than residual saturation and its volatility is low, the

greatest reduction in its saturation will be due to displacement by

water (Herbeck et al., 1976).

The increased temperatures that accompany the steam

injection process will cause decreases in the capillary and

interfacial forces between fluids and the porous media which will

reduce the residual saturation of the organic phase behind the

hot water front. Also, thermal expansion of the organic phase

can increase its saturation, increasing the mobile fraction and

decreasing the residual. Another effect of the increased

temperature is a decrease in the viscosity of the organic phase,

which also increases its mobility. All of these factors contribute

to the formation of a bank of the organic phase in front of the

steam front that is displaced by physical forces.

The main recovery mechanisms for contaminants in the steam

zone are steam distillation (also called co-distillation) and steam

displacement (stripping). Steam distillation occurs when a liquid

that is immiscible with water is present. The boiling point of the

mixture is reached when the total vapor pressure of the system

becomes equal to one atmosphere, rather than when the vapor

pressure of the individual component becomes equal to one

atmosphere. Since both liquids contribute to the total vapor

pressure, this point is reached at a lower temperature than the

normal boiling point of either of the liquids alone (Atkins, 1986).

Thus, some immiscible contaminants that have a normal boiling

point that is greater than 100°C may also be readily removed by

the steam injection process. The steam distillation process is

dependent on the composition of the liquids, as well as the

temperature and pressure of the system. Steam stripping occurs

as the injected steam sweeps the contaminant vapor to the

condensation front, where the vapors condense, increasing the

saturation of the liquid contaminant at the condensation front

and allowing additional vaporization of the contaminant in the

steam zone. When the saturation of the contaminant exceeds its

solubility at ambient temperature, a contaminant bank is formed

in front of the steam zone.

When the contaminant is a mixture of volatile and semivolatile

components, such as gasoline or kerosene, the lower boiling

components will vaporize first due to their larger vapor pressures.

As a result of the removal of the volatile components, the liquid

phase concentrations and thus the vapor pressures of the

remaining “semi” and “nonvolatile” components in the liquid

phase will increase, which leads to an increase in the evaporation

rates of these components in the steam zone (Yuan, 1990). This

sequential vaporization of compounds means that the

contaminant bank which is recovered about the time of

breakthrough of the steam will be enriched in the lighter, more

volatile components of the contaminant (Weyland et al., 1991).

By continuing steam injection after steam breakthrough, the less

volatile components can be recovered at greater rates as their

concentration in the remaining liquid, and thus in the vapor

phase, increases. Udell and McCarter (1996) showed in onedimensional laboratory experiments that compounds with boiling

points up to approximately 300°C were removed by continuing

injection of steam past breakthrough while the concentrations of

compounds with boiling points up to 450°C were reduced by at

least an order of magnitude by the injection of 100 pore volumes

of steam. However, as the volume of remaining liquid decreases

and it recedes into smaller pores, interfacial effects increase and

the vapor pressure of the contaminants decreases, and this

ultimately limits the amount of contaminants that can be recovered.

The higher the temperature, the greater the vapor pressure

(within the limits of the interfacial effects) and thus the greater the

recovery (Lingineni and Dhir, 1992).

Cycling of steam injection and vacuum extraction after steam

breakthrough at the extraction well has been found to be very

effective during field demonstrations for the recovery of

contaminants (Udell and Stewart, 1989; Newmark and Aines,

1995). Itamura and Udell (1995) have shown theoretically and

numerically that depressurizing the steam zone by halting steam

injection while continuing vapor extraction will cause a

thermodynamically unstable system. To bring the temperature

and pressure back into thermodynamic equilibrium, the

temperature must be reduced to the point where it is in equilibrium

with the reduced pressure of the system. Heat is lost by

evaporation of the residual water and contaminants, which are

then removed from the system by vacuum extraction. The

injection of steam after equilibrium has been reached at the

lower pressure allows a recharge of the heat necessary to drive

the evaporation of contaminants, and the depressurization cycle

can then be repeated. Itamura and Udell (1995) show that

cycling of steam injection with continuous vacuum extration will

always reduce the amount of steam required to meet a given

clean-up level, and may also reduce the overall clean-up time.

Another mechanism that can enhance contaminant recovery

during steam injection is enhanced desorption of contaminants

from the porous solids. For a contaminant to desorb from a solid

surface, it must absorb heat. The amount of heat that is required

for desorption to occur is dependent on both the contaminant

and the type of soil. Lighty et al. (1988) have found that

essentially all of a semivolatile contaminant could be desorbed

from glass beads and sands, but only about 80 percent of the

contaminant was desorbed at the same temperature from porous

clays and peat. The experimental results lead to the conclusion

that a monolayer of adsorbate is strongly bound to the solid

surface of a reactive soil such as a clay. The desorption of this

monolayer is a long process (Tognotti et al., 1991), and likely

requires temperatures significantly above the boiling point of the

contaminant for complete removal (Lighty et al., 1988). Lighty et

al. (1990) found that xylene, even at temperatures above its

boiling point, can still adsorb from a gas stream onto a clay

particle. The very slow desorption indicates that a very strong

bond can be formed between reactive soils and organic

contaminants.

Thus, contaminants can be recovered in the vapor phase, as

a separate phase liquid, and dissolved in the aqueous phase.

The relative amount of the contaminants in each phase will

depend on the original concentration of the contaminant and its

boiling point. Field demonstration projects of steam injection

where gasoline and diesel fuel were recovered have found that

most of the contaminant recovered was recovered in the vapor
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phase (Newmark and Aines, 1995; EPA, 1995b), while a

demonstration to recover JP-5 recovered most of the contaminant

as a liquid (Udell et al., 1994).

Although it has been shown that steam injection can potentially

recover a large percentage of volatile contaminants, it is expected

that residual amounts of the contaminants will remain in the

subsurface. These small amounts of contaminants can likely be

remediated by natural attenuation or bioremediation. Thus, it is

important to understand the effects of steam injection on microbial

populations and their ability to degrade residual contaminants.

Biological samples were taken at the Naval Air Station Lemoore

after a steam injection demonstration to recover JP-5. These

samples showed high numbers of active bacteria in the zones

that reached 80°C to 100°C for extended periods during the

demonstration and suggested enhanced biodegradation was

occurring in the heated, oxygenated soil (Udell et al., 1994).

Research carried out as part of the Dynamic Underground

Stripping project showed that before steam injection a wide

variety of microorganisms were actively degrading the BTEX

components of gasoline. Pseudomonas was the dominant

species originally, but Flavobacter was dominant after vacuum

extraction. Above the water table, the largest populations were

in areas where the contamination was at low concentrations.

Below the water table, oxygen concentrations were low and

there was effectively no microbial activity. After steam injection,

extensive microbial communities were found in all samples,

including those where the temperature had reached 90°C. The

population, however, had shifted to yeasts and related organisms

which had been observed in small numbers before heating. The

community includes thermophiles previously identified from

environments such as hot springs, and a number of organisms

apparently represent previously unidentified species. The

community includes the BTEX degrader Rhodotorula (Newmark

and Aines, 1995).

Contaminant and Soil Type Considerations

The decision to use steam injection for remediation should be

based on considerations of both the contaminant to be removed

and the properties of the porous media to be remediated. The

most important property of the contaminant in determining its

receptiveness to remediation by steam is its volatility. If a

contaminant is not volatile, a hot water displacement without the

expense of steam injection can reduce the saturation of the

contaminant to its residual saturation. For contaminants that

have very low boiling points and a large Henry’s constant, and

which are above the water table, vacuum extraction may be able

to adequately remove a significant portion of the contaminant

from sandy, homogeneous soils. Steam injection is justified for

removing trapped lenses or ganglia of a volatile or semivolatile

contaminant that cannot be removed by viscous forces. Udell

and Stewart (1990) feel that steam injection has perhaps the

greatest potential to significantly decrease clean-up time and,

therefore, offset the greater capitol costs of the system when

semivolatile contaminants are to be removed. Semivolatile

contaminants include the less volatile petroleum hydrocarbons,

such as diesel or jet fuels, and some of the higher chlorinated

solvents. However, temperatures significantly greater than

ambient temperatures may be required to desorb even volatile

contaminants, especially from clays or peat. Higher temperatures

enhance the volatilization of liquid or adsorbed contaminants

and allow greater recovery (Lingineni and Dhir, 1992). Thus,

steam injection, along with vacuum extraction, may be necessary

to achieve the desired clean-up levels for volatile contaminants

when they are present in fine soils. Most of the volatile and

semivolatile organics show very significant increases in volatility

as the temperature increases, and most have very high vapor

pressures at steam temperatures.

Stewart and Udell (1988) have shown theoretically that the

viscosity of the contaminant is important in determining the size

and saturation of the contaminant bank displaced by the steam

front. Their calculations show that steam is capable of mobilizing

any length of contaminant bank up to a contaminant/water

viscosity ratio of three. For larger viscosity ratios, the displacement

is unstable; i.e., fingering may occur, reducing the efficiency of

the displacement. Their laboratory experiments support this

theory, showing that low viscosity contaminants can be recovered

in front of or just behind the steam front, while a mineral oil with

a viscosity seven times that of liquid water at steam temperature

was not recovered by steam injection.

Soil type has been found to have a strong effect on the rate of

contaminant removal from soils (de Percin, 1991; Lighty et al.,

1988). Adsorption onto glass beads and silica sands does not

appear to form tight bonds; the adsorption is readily reversible

even at low temperatures. Experiments performed by Lighty et

al. (1988) showed that essentially all of the xylene adsorbed by

silica sands was recovered rapidly. However, for reactive

media, the desorption process is much slower, which may be

caused by a strongly adsorbed monolayer on the particle surface

(Lighty et al., 1988; Tognotti et al., 1991) or slow diffusion from

small inner pores to the surface of the particle (Keyes and Silcox,

1994).

Laboratory studies by Hadim et al. (1993) have shown that

remediation by steam injection can be achieved much more

rapidly in coarser soils than in fine soils. Their experiments have

also shown that soils which are poorly sorted (i.e., have a wider

range of grain sizes) have a lower contaminant recovery efficiency

when steam is injected. This effect is due in part to lower

permeability in fine grained or poorly sorted soils, which causes

a lower injection rate at a given injection pressure, and a greater

residual saturation. For sands within a certain range of grain

sizes, the same contaminant recoveries were achieved, but as

the grain size decreased, longer treatment times were required

to achieve the same recovery. As the grain size was decreased

further, there was a reduction in the recovery efficiency.

Heterogeneity in the subsurface can have a very significant

effect on the efficiency of steam injection. Small scale

heterogeneities can cause fingering, especially at the higher

flow rates (Basel and Udell, 1989). Channeling will occur when

layered heterogeneity exists due to naturally occurring beds or

to manmade disturbances of the subsurface such as highly

permeable trenches containing buried utilities (EPA, 1995b).

The degree of channeling that will occur increases as the

differences in the permeabilities of the layers increases, as the

ratio of the layer thicknesses (more permeable layer/less

permeable layer) increases, and as the rate of steam injection

increases (Basel and Udell, 1991). The occurrence of channeling

can allow contaminants in the low permeability zones to be

bypassed by the steam. However, there have been times when

a relatively impermeable layer overlaid the target zone for steam

injection, and the impermeable “cap” limited the vertical movement

of the steam (i.e., steam override), and thus increased its

horizontal spreading (Farouq Ali and Meldau, 1979; Aines et al.,

1992). Thus, although lenses of less permeable soils that

contain contaminants can reduce the efficiency of steam injection,

full layers of this low-permeability medium overlying the permeable

zone to be treated may actually improve the sweep efficiency of

the injected steam through the permeable zones.
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Many researchers have found that the recovery from steam

injection is related closely to heating rates (Myhill and Stegemeier,

1978; Baker, 1969; EPA, 1995b). However, this dependence is

more related to steam properties and injection rates; the thermal

properties of soils and the liquid contaminants have generally

not been found to vary enough to affect the recovery achieved by

steam injection (Myhill and Stegemeier, 1978; Hadim et al.,

1993). One property of the soil or aquifer to be treated that may

significantly affect the process is the thickness of the target zone.

In any steam injection process, some of the heat that is injected

will be lost to the overlying and underlying strata. The thicker the

target zone to be treated, the greater the percentage of the

injected heat that stays within this zone.

Another property of the media that can affect the efficiency of

the heating process is the permeability. When the permeability

is low, the injection rate at a given pressure is low, which

increases the time for heat to be lost to the overburden and

underburden, and decreases the efficiency of the overall process.

Thus, sandy media are more easily treated by steam injection

than clays. Fan and Udell (1995) studied the movement of the

evaporation front during steam injection from areas of high

permeability into beds of lower permeability. They found that the

lower the permeability of the less permeable zone, the higher the

temperature must be in that zone for evaporation to occur. The

reason for this is the decrease in vapor phase flux in the zone

with decreasing permeability, creating a higher pressure in the

zone and thus slowing evaporation.

Physical factors relating to the location and quantity of the

contaminants in the subsurface are also important. Small

volumes of contaminated soils may be more economically

treated by excavation and incineration, especially if the

contaminated soil is at or near the ground surface. The pressure

of the injected steam is limited by the overburden pressure,

which is a function of the depth of soil above the zone of injection.

When the steam pressure is greater than the overburden pressure,

fracturing of the overburden may occur, which could allow shortcircuiting of the steam to the ground surface. Thus, shallow

contamination may severelly limit the use of steam injection as

it limits the injection pressure that can be used, again pointing to

the use of excavation and incineration or thermal desorption for

contaminants near the soil surface. The steam injection method

may be a very efficient way to treat contaminated soil at significant

depths, and can be used to depths greater than 100 feet. When

the contamination is contained in a very narrow depth range, and

there is no overlying and underlying confining layers to limit

steam movement in the vertical direction, steam injection

efficiency may decrease (EPA, 1995b). Steam injected into a

fractured media would be expected to flow through the more

permeable fractures and would be ineffective for displacing

contaminants trapped in the porous matrix or in deadend fractures.

However, conductive heating from the fractures into the matrix

may effectively distill trapped volatile contaminants which could

then flow in the vapor phase to recovery wells (Udell, 1997).

When liquid phase contaminants are displaced, they are

concentrated in front of the steam zone as the steam displacement

progresses. This concentrating of liquid phase contaminants

may create a potential for downward migration of the liquid

phase contaminant, particularly if the liquid phase contaminant

is more dense than water. A confining layer beneath the zone to

be treated may help in limiting liquid phase contaminant downward

movement during steam injection (EPA, 1995b).

Design Considerations

One of the most important design considerations for a steam

injection process is the steam injection rate. This factor, however,

cannot be separated from considerations of the injection pressure,

temperature, and steam quality. The injection pressure is limited

by the depth of injection: when injection pressures greater than

the overburden pressure are used, fracturing can occur in the

overburden, allowing steam to escape to the surface. Udell et al.

(1994) recommend that the injection pressure be as high as

possible while not exceeding the soil fracture pressure which is

estimated as 1.65 psi per meter of depth below the ground

surface. Thus, the shallower the zone to be treated, the lower the

injection pressure must be. There is a direct relationship

between the injection pressure and injection rate, which are

related by the permeability of the media. The more permeable

the media, the greater the injection rate that can be achieved at

a given limiting pressure. The greater the injection rate, the

greater the heating rate of the subsurface. Newmark and Aines

(1995) recommend large amounts of steam for establishing a

complete steam zone in very permeable media. However, once

the steam has reached the production well, the higher the steam

injection rate the more steam (and therefore heat) that is produced.

Thus, once steam breakthrough has occurred, greater thermal

efficiency can be achieved by reducing the steam injection rate

(Myhill and Stegemeier, 1978). Newmark and Aines (1995)

found that repeated steam passes were effective for heating

small impermeable layers in between steam zones. The optimum

injection rate is dependent on many variables specific to a given

system, such as distance between wells, sweep efficiency, and

heat losses to over- and underburden, and may be best

determined by field experimentation (Bursell et al., 1966).

Greater heating rates generally mean greater recoveries, and

greater energy efficiency. Increasing the temperature of the

injected steam, however, does not necessarily increase the rate

of heating of the target area. Johnson et al. (1971), studying

steam displacement of oil from a reservoir, found that when they

increased the temperature of the steam they ultimately recovered

more oil, but the greater temperature gradient between the

steam injection zone and the over- and underburden increased

the rate of heat loss, thus the steam front actually advanced more

slowly. Longer times were required to achieve the increased

recovery, and the additional oil recovered at the higher

temperatures required a disproportionately larger amount of

steam. Baker (1969) found that for a given steam injection rate,

the rate of heat loss to the overburden and underburden increases

with time as the area of contact with them grows. At some point,

the rate of steam input will equal the rate of loss to the over and

underburden, and the growth of the steam zone will stop. Thus,

there is a maximum size to the steam zone for a given injection

rate.

Several researchers have found a direct relationship between

the steam quality and the oil/steam ratio at the producing well.

Steam quality is defined as the proportion of the total water that

is in the vapor phase; if liquid water is not present, the steam

quality is 100 percent. Myhill and Stegemeier (1978) and

Singhal (1980) found that as the quality of the steam at the

injection point increases, the oil/steam ratio in the extraction well

increases.

Pilot- and full-scale steam injection demonstration projects

seem to indicate that greater efficiency is achieved by using

continuous steam injection; i.e., 24 hours a day and at least 6
6

days per week, rather than intermittent injection (16 hours per

day, 5 days per week). The intermittent injection schedule allows

the soil to cool somewhat during the 8 hours when steam is not

being injected, significantly slowing the overall heating rate and

maximum temperatures in the target zone. Intermittent operation

also was found to put additional stress on the boiler and other

process equipment due to the frequent cooling and heating, and

caused additional system down time (EPA, 1995b).

A physical characteristic of steam injection processes that

must be considered is steam override due to gravitational forces.

Gravity override is caused by the fact that steam is much less

dense than liquids and therefore will tend to rise in the porous

medium. At the same time, hot water produced by the

condensation of steam is more dense than some contaminants,

so may tend to underride the zone to be treated (Singhal, 1980).

A frontal displacement mechanism with essentially pistonlike

flow will displace more of the initial fluids by viscous forces.

When steam override occurs, the effectiveness of the frontal

displacement is reduced, and thus the vertical sweep efficiency

of the steam displacement process is reduced. Various properties

of the system determine the amount of override that occurs.

When steam is injected into an unsaturated porous medium the

steam front is essentially vertical. When there is a difference in

density between the injected fluid and the fluid initially present,

the slope of the interface between the fluids is equal to the

difference between the vapor and liquid water phase viscous

forces divided by the gravitational forces. The degree of gravity

override increases as the difference in density between the liquid

and vapor phases increases, as the permeability of the medium

decreases, and as the viscosity of the liquid phase being

displaced increases. One operational parameter that affects

override is the steam injection rate. Steam override cannot be

eliminated, but by increasing the injection rate of the steam, the

difference between the vapor and liquid viscous forces is reduced

and thus the amount of override can be decreased (Basel and

Udell, 1989).

The major equipment requirements for a steam injection

system are the steam generator, the distribution system to the

wells, the extraction system, and the coolers/condensers for the

extracted fluids. Means of treating the off gases, the water that

is extracted, and any organic phase that is recovered will be

required, but these systems are beyond the scope of this paper

and will not be discussed further. Mobile steam plants are

available (Newmark, 1992) that are powered by natural gas,

propane, or another available fuel. Steam generators require a

high quality feed water to avoid scale buildup in the generator

(Schumacher, 1980), so often the generator feed water must be

treated before use. Normally, operation of a steam injection

system will require that the boiler be operated and manned

continuously during the injection process (Newmark, 1992).

Pneumatic air lift pumps (Newmark, 1992) and jackpumps

(Udell and Stewart, 1989) have been used in the extraction wells

to remove liquid water and contaminants. Blowers are used to

remove steam and contaminants in the vapor phase. Heat

exchangers are used to condense the extracted vapors and to

preheat the steam generator feed water.

Special consideration must be given to the construction of the

steam injection wells due to the high temperatures and pressures

they must withstand. PVC or fiberglass wells that are commonly

used for ground-water monitoring are not adequate for steam

injection processes. Steel casing is commonly used for both

injection and extraction wells, but the installation must allow for

the expected expansion of the casing with temperature.

Experience in the oil industry has shown that pipe failures in

steam injection projects occur at the couplings, due to repeated

expansion and contraction during the heating and cooling

processes (Gates and Holmes, 1967). Cement used for

conventional wells will frequently fail when exposed to the high

temperatures associated with the steam injection process

(Schumacher, 1980). The addition of 30 to 60 percent by weight

of quartz silica or silica flour to conventional cements will provide

temperature stability, and the addition of sodium chloride will

allow the cement to expand linearly with temperature (Gates and

Holmes, 1967). Information on well construction casing materials

and cements that have worked for steam injection wells is

available from both the oil industry (Gates and Holmes, 1967;

Hall and Bowman, 1973; Farouq Ali and Meldau, 1979; Chu,

1985) and from pilot-scale contaminant recovery operations

(Newmark, 1992).

The placement of steam injection and extraction wells is

critical to the efficiency of the remediation system. Determining

well placement and injection zones should be based on a

thorough knowledge of the subsurface characteristics, including

delineation of zones of high permeability and high contaminant

concentration. If the target area to be remediated is not too large,

injection wells may circle the contaminated zone with one or

more extraction wells in the center. When the area to be

remediated is large enough that the desired injection rates

cannot be achieved by peripheral injection wells, injection and

production wells are usually arranged in a pattern. The 5-spot

pattern (see Figure 3) alternates injection and extraction wells so

that all wells are surrounded by four wells of the opposite type.

Patterns using two injectors per producer (7-spot pattern, see

Figure 3) have also been used by the oil industry to recover

viscous oils (Boberg, 1988). Patterns such as these may be

effective when the subsurface and contaminant distribution is

essentially homogeneous. When significant heterogeneity exits,

well placement must take this into account. When a pronounced

directional permeability trend exists, this will control well

placement. Edge or peripheral drives may be more appropriate

when there is significant geologic structure (Willhite, 1986).

Farouq Ali and Meldau (1979) recommend close well spacings

and an ample number of production wells, and Udell et al. (1994)

found at a field demonstration that 5 to 6 meter well spacings

appeared to work better than 12 meter well spacings. Based on

Figure 3. Five spot and seven spot well patterns used for steamflooding.
7

the results of this demonstration, they also recommended that

flexibility be built into the system by installing wells that can be

used for either steam injection or fluid extraction. The heat

losses observed by Baker (1969) would suggest that there is a

maximum radial area that can be heated by a given steam

injection rate, and this would set an upper limit on distance

between wells in order to heat the entire area. Pilot studies for

contaminant recovery have used well spacings as low as 1.5

meters, but full scale operations have used well spacings on the

order of 18 meters (Aines et al., 1992; Newmark, 1992; EPA,

1995b).

The sweep efficiency of the injection process is the areal and

vertical amount of the formation targeted for treatment that is

actually contacted by the steam. Sweep efficiency can be

reduced by areas of lower permeability that are bypassed by the

steam. When large areas of the formation contain lower

permeability materials, steam may essentially miss the whole

area. One technique that has been used successfully to increase

sweep efficiency is to shut down the production wells which have

a good connection to the injection well once those areas are

clean (i.e., are no longer producing contaminants), and to

continue producing from wells that are in the direction in which

steam penetration is desired (Powers et al., 1985; EPA, 1995b).

Where a high viscosity fluid is to be recovered it may be desirable

to heat a large portion of it early on to reduce its viscosity. This

has been accomplished by injecting steam into a lower more

permeable layer, and allowing heat to transfer upward to the oil

and increasing its mobility (Hall and Bowman, 1973). In a case

where a viscous oil was floating on top of the water table, steam

injection below the water table was found to conduct heat into the

oil layer to increase its mobility (Farrington and Sword, 1994).

Gravity override may also aid in distributing heat to fluids of low

mobility. Steam may spread evenly on top of a contaminant layer

and conduct heat downward to heat the contaminant and increase

its mobility through viscosity reduction and/or distillation (Farouq

Ali and Meldau, 1979).

Effluent stream monitoring is required to monitor the progress

of the remediation process. Vapor and aqueous phase samples

are normally collected at regular intervals during the course of a

remediation and analyzed to determine the amount of

contaminants being removed. Sample intervals of one hour

have been used on a small-scale demonstration project, while a

one-day sampling interval was used on a full- scale demonstration.

However, the time required to analyze these samples by

techniques such as gas chromatography limits their usefulness

for process control and optimization purposes. Also, it has been

found that effluent streams, particularly the vapor effluent stream,

can vary significantly over short periods of time, and these

variations cannot be monitored with the one-day or even onehour periods between samples that are generally used for grab

samples. Flame ionization detectors have been used with some

steam injection systems for real time monitoring of the

contaminants being recovered in the vapor phase (de Percin,

1991; EPA, 1995b). Fourier transform infrared radiation (FTIR)

(Langry and Kulp, 1994) and differential ultraviolet absorption

spectroscopy (DUVAS) (Barber et al., 1994) have both been

evaluated for real time monitoring of effluent vapors. FTIR

measures alkane components in the vapor stream, while DUVAS

measures aromatic compounds. To date, the concentration

information from these on-line monitors has been more qualitative

than quantitative, but these monitors have been shown to

provide a real time estimation of the amount of hydrocarbons

being recovered by the system. In order to correctly interpret the

effluent concentration data, it must be kept in mind that a lag time

between injection and the effect on the extraction well is to be

expected. Also, mass transfer limitations can occur within the

subsurface, which reduces effluent concentrations and makes it

appear that the concentration remaining in the subsurface is at

lower levels than it actually is. Thus when the extraction system

is shut down for a period of time and then restarted, effluent

concentrations are higher than they were before the shutdown.

Tracking the movement of the steam injection front in the

subsurface is also desirable for monitoring the progress of the

process, and to aid in understanding the processes that are

occurring as a result of steam injection. Temperature

measurements, either at intermediate observation wells or at the

producing wells, provide a direct means of tracking steam front

movement (Hall and Bowman, 1973; Powers et al., 1985;

Newmark, 1992). However, when additional steam passes are

used after the subsurface is heated to approximately steam

temperature, the small differences in temperature produced by

the passage of an additional steam front may be hard to detect.

Thus, other means of tracking steam front movement may be

needed (Newmark and Aines, 1995). Powers et al. (1985) also

monitored the chloride concentration in the naturally occurring

brine that was produced, and found measurable decreases in its

chloride content due to dilution by condensed steam before a

thermal response was detected. Neutron logs will indicate the

presence of vapor saturation and, thus can be used to monitor

the steam front movement (Hall and Bowman, 1973). Radioactive

tracers have also been used to determine which injection wells

are influencing each of the various producers and to what extent

(Powers et al., 1985).

The Dynamic Underground Stripping Project (Newmark, 1992;

Newmark and Aines, 1995) used a wide variety of geophysical

techniques both before steam injection to establish baseline

information and to help characterize the subsurface, and during

steam injection to test their ability to monitor the movement of the

steam front. Geophysical logs were compared to the temperature

records obtained from monitoring wells to provide additional

insight into the results from the geophysical logs. Low

permeability, clay-rich zones have relatively large amounts of

exchangeable cations, giving them a relatively lower electrical

resistivity than more coarse-grained soils. Electrical resistance

tomography (ERT) provides cross-sections of subsurface

resistivity and can identify the higher permeability zones where

steam flow is likely to occur. Temperature increases may

increase the mobility of exchangeable cations and thus decrease

resistivity, or in areas where the steam has caused desaturation,

the resistivity may increase (Vaughan et al., 1993). Thus,

changes in resistivity were found to be useful for mapping the

movement of the steam front (Ramirez et al., 1993). Induction

resistivity logs measure the resistivity of the pore fluids near the

boreholes providing a more detailed vertical view of steam in the

subsurface, and were found to delineate the vertical temperature

distribution. Tiltmeters, which measure deformations in the

ground surface that result from a pressure transient in terms of

tilt, also showed potential for determining the movement of the

steam front. Tiltmeters were found to be more useful after the

ground was already heated to approximately steam temperature

to map the movement of additional steam passes (Newmark and

Aines, 1995).

Numerical models may be useful as an aid to designing cleanup operations using steam injection, and to provide estimates of

the time required to achieve different levels of contaminant

removal (Newmark, 1992). Many models of heat flow (Spillette,

1965; Marx and Langenheim, 1969; Prats, 1969) and oil recovery

from a reservoir due to steam injection (Willman et al., 1961;
8

Vinsome, 1974; van Lookeren, 1983; Rubin and Buchanan,

1985) have been developed. Because there are significant

differences between steam injection for oil recovery and for

contaminant removal, numerical models developed for oil

recovery processes would not necessarily include all of the

processes that are important in the contaminant remediation

process. Partitioning processes between the air, water, and

solid phases that occur at the pore level are not generally

important in oil recovery operations, but may be very important

when considering removing contaminants down to the parts per

million and parts per billion level (Falta et al., 1992a). The

thermal model developed by Rubin and Buchanan (1985) is

general in its formulation and may be adequate to model some

aspects of a steam injection process for subsurface remediation.

A finite-difference simulator called the Multicomponent Multiphase

Nonisothermal Organics Transport Simulator (M2

NOTS) has

been developed by the University of California at Berkeley to

simulate the steam injection process for the purpose of subsurface

remediation. This simulator was used to aid in the design of the

Dynamic Underground Stripping Project, however, comparisons

between simulations and the field trial at the gasoline spill site

are not currently available.

Laboratory Experiments

Two-dimensional laboratory experiments on steam injection

have also been done. When steam was injected into a twodimensional homogeneous sand pack containing o-xylene, the

xylene was displaced as a free product bank ahead of the steam

condensation front. Essentially complete recovery of the

contaminant was achieved with the injection of approximately

350 pore volumes of steam. However, when steam was injected

into a layered sand pack containing diesel fuel, there was not a

significant physical displacement of the diesel by the steam

condensation front. Instead, the volume of the initial mobile pool

decreased as it was displaced by the steam front, and residual

hydrocarbon liquid was observed in all regions that had contained

the diesel. Continued steam injection and vacuum extraction

then produces a fractional distillation of the diesel fuel components

with the most volatile components being removed first (Udell,

1994).

Itamura and Udell (1993) studied the recovery of

tetrachloroethylene (PCE), a dense, semivolatile contaminant,

from a two-dimensional sand pack that contained mainly layered

heterogeneity. Near the top of the model and above the water

table was a layer containing diamond-shaped lenses of varying

permeability sands. The contaminant was introduced at the top

of the model and the contaminant that reached the zone of

variable permeability lenses preferentially migrated to the lowest

permeability zones in this region and then along the boundaries

between the different permeability sands. Steam was injected

over most of the vertical depth of the model, invading the most

permeable layer first, and then the variable permeable layer

containing most of the contaminant. When the injected steam

reached the contaminant, it was displaced both downward and

horizontally towards the recovery well. The mobile contaminant

in front of the condensation front moved along the boundary

between different permeability sands. After the mobile, separatephase contaminant had been displaced, the contaminant

continued to be recovered in the vapor phase. The highest

recovery rate of PCE occurs just before steam breakthrough in

the regions containing the majority of the contaminant. Visually

it could be determined that some of the contaminant remained in

the model, but the percent of the contaminant recovered was not

reported.

Field Trials

Table 1 contains a compilation of some of the details of steam

injection field trials that have been carried out. In The Netherlands,

steam stripping has been used to remediate contaminated soils

since at least 1983. However, only limited information on the

applications of steam stripping could be found in the literature.

Hilberts (1985) describes a vacuum bell structure that is placed

on top of the soil and is commonly used for the steam injection

process. The 2 meter by 2 meter box contains 4 lances for steam

injection, and has been used to treat contaminated soils as deep

as 4.5 meters. Contaminants are drawn off from the center of the

bell, which creates a vertical flow of the steam up through the soil.

At the Broomchenie site which contained organic bromides,

reductions of 97 percent in the most heavily contaminated soil

were found, but further analysis showed that most of the organic

bromide had been converted to inorganic bromide which remained

in the soil. Only a small amount of bromide was present in the

condensate removed from the soil (Hilberts, 1985).

A small-scale pilot demonstration project was undertaken at

the Solvent Services, Inc. site (San Jose, California) in August

1988 (Udell and Stewart, 1989). Surface spills and leaking

underground storage tanks at this industrial facility had released

a variety of volatile organic compounds to the subsurface.

Extensive temperature monitoring showed that the movement of

the steam zone was controlled. Cycling of vacuum extraction

and steam injection was then carried out sporadically after

steam breakthrough and significantly greater recovery rates

were achieved at the beginning of the second vacuum extraction

period. Comparison of soil core analysis done before and after

the pilot remediation study indicates that there was some

downward migration of contaminants in solution in the

condensate. Low permeability zones isolated contaminants,

and the post-treatment concentrations in some of these zones

was higher than the initial concentrations. The authors postulated

that high water saturations containing high concentrations of

contaminants which were pushed in front of the steam front were

imbibed into the low permeability areas and caused the observed

increases in the more highly water-soluble contaminants. Overall,

the pilot study demonstrated the potential for steam injection in

conjunction with vapor extraction as a rapid and effective

remediation technique.

de Percin (1991) reported on a demonstration project for the

“Detoxifier” system made by NovaTerra, Inc. This system is

similar to that used in The Netherlands in that it is a shroud or bell

placed on the soil surface in which a vacuum is maintained to

remove the contaminant vapors. In this case, instead of lances

to inject steam, the steam, along with hot air, is injected through

augers that are rotating in opposite directions to break up the

soil. The steam and hot gases carry the contaminants to the

surface. The augers can treat 2.5 m3

 of soil to a depth of 8.2

meters. Blocks of soil are treated sequentially, and treatment

time is varied dependent on the soil type and contaminant

concentration. A flame ionization detector measures the organic

concentration of the gases in the shroud, which are roughly

correlated to the concentration of organics remaining in the soil,

and can indicate when a desired level of treatment has been

achieved. A tracer study showed that the mixing action of the

Detoxifier auger does not produce a homogeneous treatment

area, thus all of the soil may not receive adequate treatment.

Basile and Smith (1994) reported on a combined anaerobic

degradation/steam injection/vacuum extraction system being

used to remediate separate and aqueous phase chlorinated

hydrocarbons. Laboratory data showed that elevated bioactivity
9
10

Table 1. (continued)
11

Table 1. (continued)
12

occurred at temperatures between 30°C and 40°C, so nutrients

are injected at 40°C to promote biological dechlorination of the

contaminants. The authors claim that the degradation products

are more easily extracted than the chlorinated solvents, and are

then steam stripped and removed from the subsurface via

shallow vacuum extraction wells. They claim that the in situ

remediation was favored over excavation and off-site disposal

by local residents because it eliminated the need to transport

contaminated soil through the neighborhood.

The Dynamic Underground Stripping Project is a joint venture

between the University of California-Berkeley and Lawrence

Livermore Laboratory. The process combines steam stripping in

permeable layers and electrical resistance heating of clay layers

to heat the subsurface. Characterization of the site before steam

injection indicated that about half of the contaminants were

above, and half were below the water table. The highest

gasoline concentrations were found in the capillary fringe

(Newmark and Aines, 1995). Electrical resistance heating of the

clay layers was initiated at night (to limit interference with

construction activities going on during the day) for 12 weeks prior

to steam injection, raising the temperature in the clays from 20°C

to 50°C. Steam was injected both above and below the water

table, and during 5 weeks of continuous steam injection over

6,400 liters of gasoline was recovered, most of it in the vapor

phase.

After about 3 months, the second phase of steam injection was

begun. During this phase, steam was injected intermittently, so

that the treatment zone was kept desaturated and periodically

depressurized. Cycle times were 5 to 6 days, and this phase was

continued for 6 weeks. Extraction of gasoline was found to be

particularly vigorous when the treatment zone was maintained in

a depressurized condition at elevated temperatures, and a total

of 18,500 liters of gasoline were recovered. Soil cores taken

after the second phase of steam injection showed that most of

the gasoline remaining in the subsurface was trapped in the low

permeability zone between the two steam zones. A final phase

of vapor extraction with limited electrical heating and periodic

fluid extraction was conducted for 3 months, which recovered an

additional 3,800 liters of gasoline (Yow et al., 1995). It is believed

that no additional free gasoline remains in the treatment zone,

but benzene and toluene remained in the ground-water at

concentrations above their maximum contaminant limits.

Maximum contaminant limits had also been set for 1,2-

dichloroethane and xylenes, and these levels had been met.

Biological sampling at the conclusion of the project showed that

BTEX degraders had survived the heating and could rapidly

remove the remaining contaminants from ground-water (Newmark

and Aines, 1995).

As part of EPA’s SITE demonstration program (EPA, 1995b),

a steam injection demonstration was carried out at the Rainbow

Disposal site in Huntington Beach, California, between August

1991 and August 1993. The soils treated by this system were

under and around existing facilities at the transfer facility, and

transfer activities continued during the remediation efforts. The

discontinuous clay layers at this site created a nonuniform

distribution of the contaminant in the subsurface. The spilled

diesel flowed downward through the sand layers, and when it

reached a sand/clay interface, it flowed laterally along the

interface until it reached a break in the clay layer which allowed

it to flow downward by gravity again. This caused large variations

in concentrations even over small vertical distances.

Maintenance of the boilers, the oxidizing unit to treat offgases,

and the steam distribution system caused considerable downtime,

and operating logs from the project showed the system had an

online factor of 50 percent. Initially the steam injection system

was operated 16 hours per day, 5 days per week. This led to

inefficient heating of the soil and maintenance problems with the

boilers due to the thermal stress associated with frequent

startups and shutdowns. An increase in the soil heating efficiency

and diesel recovery was noted when a 24 hours per day, 6 days

per week schedule was adopted, and the operation of the boilers

improved.

The clean-up criteria set by the California Regional Water

Quality Control Board was 1,000 mg/kg of total petroleum

hydrocarbons (TPH). It appears that this criteria was not met, as

45 percent of the post-treatment samples inside the treatment

zone were above this level. For the small number of soil cores

available for which there is pre- and post-treatment data, it

appears that the diesel fuel moved downward in the time

between coring and steam injection. The post-treatment soils

data shows the majority of the contamination is in the range of 8

to 12.2 meters in depth. This could have been caused by the loss

of a perched water table due to the very dry conditions in the

region. Steam was injected at 10.7 to 12.2 meters below ground

surface, and the 7.6 meter long screens on the extraction wells

pulled vapor and liquids from the 3 to 10.7 meter depth range.

Thus, the steam had a vertical component of flow, caused by the

vertical positions of the screens on the injection and extraction

wells. The growth of the steam zone was monitored by a limited

number of temperature wells, and this data shows that the region

from 6.1 to 9.1 meters below ground surface was generally

heated to steam temperature. At the bottom of the wells (12.2

meters), the temperature generally did not rise above 70°C.

Some of the highest concentrations of TPH were in an area

where underground storage tanks existed, and this area only

received steam for a short period of time at the end of the project.

Thus, it appears that a majority of the remaining contamination

(but not all) was in areas that did not reach steam temperature.

Also, cycling of steam injection and vacuum extraction, which

was found to be very effective at other field sites for recovering

contaminants, was not used at this site.

 A more thorough understanding of the subsurface conditions,

a better designed injection/extraction system to heat the lowest

zones where contaminants occurred, more temperature

monitoring wells throughout the treatment area and monitoring

of the steam injection rate at individual injection wells to monitor

the growth of the steam zone, and more effective operation may

have improved the overall effectiveness of this system.

A pilot steam injection study was carried out at the Yorktown

Naval Shipyards in Yorktown, Virginia (Farrington and Sword,

1994), where Naval special fuel oil had leaked from underground

storage tanks. This oil is slightly less dense than water, with a

specific gravity in the range of 0.94 to 0.99 at ambient

temperatures, and is essentially insoluble in water and nonvolatile.

The viscosity of this oil, measured on two different samples,

ranged from 137 to 50 centipoise at 38°C. Thus, the oil at this site

is floating on top of the shallow water table and moves only slowly

at ambient temperatures. Steam was injected below the water

table to heat the oil, and oil began to be recovered from the

system when the average temperature reached about 60°C.

The viscosity of the oil at this temperature is in the range of 20

to 40 centipoise, a substantial reduction from the ambient

temperature viscosity. For an oil which is nonvolatile, and where

the injected fluid is not being used to physically displace the free

phase liquid, hot water injection, if implemented correctly, may

be capable of recovering as much of the free-phase oil as steam

injection without the associated difficulties and extra costs of

generating and injecting steam.
13

A pilot scale demonstration project was carried out at Naval Air

Station Lemoore in Lemoore, California, in 1994 to recover JP5 (Udell et al., 1994). The system consisted of two injection wells

surrounded by 8 extraction wells within the contaminated zone.

Thus, much of the recovered fluids, including an estimated 80

percent of the recovered JP-5, was from outside of the targeted

treatment area enclosed by the extraction wells. Concentration

of TPH inside the treatment area in the vadose zone was

reduced from as high as 100,000 ppm to 20 to 50 ppm. At and

below the water table, however, TPH concentrations remained

around 20,000 ppm. Soil samples taken after 35 days of steam

injection showed high TPH concentrations at the interface of the

surface clay layer and the underlying silty sand which had

previously appeared to be uncontaminated. Continued steam

injection reduced these concentrations. Recommendations for

reducing or eliminating the upward spreading of contaminated

vapors included control of the ground-water elevation to avoid

mounding during steam injection, higher JP-5 extraction rates in

both the liquid and vapor phases, and greater energy input rates

to avoid condensation. These recommendations could be

achieved by installing wells made for both steam injection and

extraction of liquids and vapors, allowing greater system flexibility

and by continuous monitoring of ground-water elevations to

optimize fluid removal rates.

A demonstration of a Dual Auger Rotary Steam Stripping

system by In-Situ Fixation, Inc., (DOE, 1997) was carried out at

the Pinellas Plant Northeast Site at Largo, Florida, starting in

December 1996. The system used was similar to the previously

described “Detoxifier” system. The remediation goal of this

demonstration project was to reduce high contaminant

concentrations (500 to 5000 ppm) to levels which are more

amenable to anaerobic bioremediation, which was the chosen

remedial technique for this site. Although 75 to 95 percent

removals of contaminants from soil and ground water were

generally achieved, the treatment goal of 100 to 200 ppm

remaining was not generally met. In the more highly contaminated

areas, the system was severely limited by the vapor treatment

system which could not treat the large quantities of volatile

organics that were released. Thus, it is difficult to fully assess the

effectiveness of the auger/steam injection system for releasing

contaminants from soil and ground water because of the limitations

placed on system operation from the undersized vapor destruction

system.

Cost Information

Cost information on innovative remediation methods is sparse

because of the limited experience with applying these methods

in the field. Because each site is unique in terms of the

subsurface geology, the types and quantities of contaminants,

and in terms of other site characteristics that may affect an in situ

treatment technology such as steam injection, costs incurred

during the application of this technology at one site are not

directly applicable to the use of the same technology at another

site. Also, much of the available cost data were collected during

demonstration tests, and clearly some of the costs incurred

during research and development of the technology will not be

incurred again as the technology is applied at other sites (Evans,

1990).

Often cost information for in situ remediation is put in terms of

a cost per unit of soil treated, and the terms of cost per cubic

meter of soil treated will be used in this Issue Paper. However,

the limitation of reporting cost data in this manner must be

understood. Some costs associated with the technology, for

example, mobilization, startup, and demobilization, are fixed

costs and are not dependent on the volume of soil to be treated.

Other costs, such as utilities for steam generation, are very

dependent on the volume of soil to be treated. Thus, the cost in

terms of per cubic meter of soil to be treated are going to

decrease as the fixed costs are spread over a greater volume of

soil to be treated. Also, consider the question of depth of the

contaminated zone to be treated. The deeper the zone of

contamination, the greater the drilling and well installation costs,

but this may not significantly increase the cost per unit of soil

treated. However, as the zone of contamination gets deeper,

costs for excavation will increase significantly, and the relative

costs of an ex situ treatment which relies on excavation of the soil

versus an in situ treatment such as steam injection are going to

change.

The costs associated with the “Detoxifier” demonstration at

the Annex Terminal Port of Los Angeles have been estimated to

be $330/m3

 to $415/m3, based on a volume of 6,824 m3 to be

treated. The range in costs is related to the percent time on line

that is used to make the estimate. Forty-four percent of the total

costs were labor costs. Based on the company’s operations to

that time with the Detoxifier, they estimate the costs to treat 9,175

m3

 of material to be $425/m3 at the treatment rate of 3.8 m3/hour,

$260/m3 at the treatment rate of 7.7 m3/hour, and $165/m3 at a

treatment rate of 15 m3

/hour. They expect the costs to reduce as

they gain additional experience and make modifications to their

equipment. These numbers show that unit costs are very

sensitive to the treatment rate, which is determined by site

characteristics, contaminants present, and the clean-up criteria

(EPA, 1991).

For the steam enhanced remediation project at the Rainbow

Disposal site, the costs associated with the project were

approximately $4.4 million. The total amount of soil that was

considered to have undergone treatment was 72,600 m3

, which

gives a cost per cubic meter of approximately $60. Labor

accounted for about one-third of the total cost, and utilities

accounted for another large portion. For this project, mechanical

problems reduced the on-line factor to about 50 percent, which

drove up the total costs, especially labor costs. It is reasonable

to expect higher on-line factors at commercial remediation sites.

Cost estimates were developed based on this project using online factors of 75 and 100 percent, and the associated costs were

estimated to be $47 and $38/m3

, respectively.

In the Dynamic Underground Stripping demonstration project,

steam injection heating costs were approximately $2.0/m3 (Udell,

1997), while electrical resistance heating costs were $6.5/m3

(Newmark, 1992). Overall costs to clean up the gasoline spill site

were $11 million, which works out to a unit cost of about $78/m3

to $92/m3, and they expect the costs to drop by at least one-half

as experience with the process is gained. These costs were also

inflated due to the fact that this was a research project that

involved a large number of researchers, and several experimental

techniques were investigated which, while adding to the overall

costs, did little or nothing to improve the efficiency of the project.

Despite the inflated costs of this project, the total costs compare

very favorably with estimated costs of $25 million for vacuum

extraction and water table lowering without steam injection, and

$30 million for excavation and backfilling (Yow et al., 1995).

Costs of the Dual Auger Rotary Steam Stripping demonstration

project totaled $919,650 and a total of 1566 m3

 of soil were

treated, giving overall treatment costs for this demonstration of

$587/m3

. However, significant downtime was experienced during

the early part of this demonstration due to equipment problems

and, as mentioned previously, the rate of treatment in some
14

areas was slowed significantly due to the undersized vapor

treatment system. With improved on-line times and when higher

treatment rates can be achieved, it is estimated that treatment

costs as low as $88/m3

 may be possible.

Conclusions

Laboratory studies and field demonstrations have

demonstrated the ability of steam injection to effectively recover

volatile and semivolatile contaminants from the subsurface.

However, in order to effectively and efficiently apply this process,

it is important to characterize the site adequately to determine

the horizontal and vertical distribution of the contaminant, and

the preferred flow paths for the injected steam. This information

is critical to the design of the steam injection and extraction

system. Effective operation of the system will likely include

cyclic operation of steam injection and vacuum extraction after

steam breakthrough at the extraction well has occurred.

Advantages of steam injection over other remediation techniques

include the fact that excavation is not required, potential

contaminants are not injected to the subsurface, and potentially

much more rapid remediations are possible. Without a doubt,

the initial capital costs for steam injection are higher than those

for a system that relies on removal of soil gases without heating,

such as vacuum extraction. However, the accelerated removal

rates can lower the total cost of cleanup by reducing the time

required for the remediation, thus reducing the overall operating

costs (Udell and Stewart, 1989). In addition, the higher

temperatures can increase the amount of semivolatile organics

that are recovered and the removal efficiencies from clay soils by

increasing the volatilization and desorption from soil surfaces. In

systems where the volatilization is limited by a low volatility of the

contaminant or strong adsorption onto a solid phase, the

temperature of the system may actually determine the clean-up

level that can be attained. There will undoubtedly be trade offs

between the efficiency of the cleanup and the cost of the

treatment process.

Notice

The U.S. Environmental Protection Agency through its Office

of Research and Development funded the research described

here. It has been subjected to the Agency's peer and

administrative review and has been approved for publication as

an EPA document. Mention of trade names or commercial

products does not constitute endorsement of recommendation

for use.

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